The present invention relates to communications apparatus and methods, and more particularly, to transmitter circuits and methods of operation thereof.
Transmitter circuits used in radio communications devices, such as wireless telephones, wireless base stations and similar apparatus, typically include an oscillator circuit that generates a periodic signal that is modulated by an information-bearing signal. The modulated signal typically is amplified by a power amplifier circuit, which produces an amplified signal that is transmitted into a communications medium via an antenna.
For example, a conventional xe2x80x9cdirect modulationxe2x80x9d transmitter circuit may include a master oscillator circuit, such as a voltage controlled oscillator (VCO) circuit, that generates a master oscillator signal that is modulated by an information-bearing signal, for example, at the master oscillator circuit itself or downstream of the master oscillator circuit. The modulated signal may be amplified by a power amplifier circuit and transmitted from an antenna, such that the transmitted output signal has the same frequency as the master oscillator signal. A potential drawback of such a transmitter structure is that the output signal from the power amplifier may leak back from the antenna to the master oscillator circuit. This may disturb the phase of the master oscillator signal and degrade the spectrum and/or Error Vector Magnitude (EVM) of the transmitted output signal.
One conventional approach for reducing such undesirable effects is to improve electrical isolation between the power amplifier output and the master oscillator circuit. Another conventional approach includes making the master oscillator signal have a frequency that is an integer multiple of the output frequency, with the master oscillator signal being divided down to the output frequency at some point before generation of the output signal. Yet another conventional approach includes using a master oscillator frequency that is ⅔ of the output frequency, with the master oscillator signal being divided by 2 and added to itself to make the final output frequency.
Each of these approaches may have drawbacks. Relying on isolation between the power amplifier output and the master oscillator circuit may produce uncertain and variable results. Using a higher frequency master oscillator signal that is subsequently divided down may result in additional noise that may not be offset by the frequency divider and may lead to excessive power consumption. Generation of multiple frequencies may also increase the likelihood of spurious signal components in the output signal.
In embodiments of the present invention, an oscillator signal having a first frequency is generated. A frequency-doubled modulated output signal is produced from the oscillator signal, the frequency-doubled modulated output signal modulated according to a modulation signal and having a second frequency that is about twice the first frequency. The frequency-doubled modulated output signal is amplified and transmitted in a communications medium. In some embodiments, the frequency-doubled modulated output signal may be generated from an oscillator signal using a frequency doubling vector modulator circuit. In some other embodiments, the oscillator signal may be phase-modulated, and the frequency-doubled modulated output signal may be generated from the phase-modulated oscillator signal by a frequency-doubling amplitude modulator circuit. The invention may be embodied as apparatus or methods.